1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically to such an image forming apparatus that an image is recorded on a rotating image recording medium and further the recorded image is transcribed onto an image transcribing medium fed in the same direction as the rotative direction of the image recording medium.
2. Description of the Prior Art
Conventionally, various image forming methods are adopted for printing apparatuses (e.g., printer) for forming an image on an image forming medium (e.g., paper). For instance, the typical methods are: (1) electrophotographic method such that an image is formed electrostatically on a photosensitive drum, the formed image is changed so as to be visible by use of toner, and then the toner image is transcribed onto a paper as an image; (2) an ink jet method such that ink is directly jetted onto a paper to form an image; (3) a silver halide photographic method such that an image is exposed and recorded on a photosensitive color forming substance; etc.
In the case where a color image is formed by use of a printing apparatus based upon the electrophotographic method, this electrophotographic method is further classified into some methods as follows: (1) A monochromatic (of the three primary colors of yellow, magenta and cyan and in addition black in some cases) toner image is formed on a photosensitive drum and the formed toner image is transcribed onto a paper. The above-mentioned process is repeated three times for three colors. (2) Three-color toner images are formed on a photosensitive drum, and the formed toner images are transcribed onto a paper simultaneously. (3) Four image forming units (for three primary colors and black) each for forming a monochromatic toner image are arranged in the paper feed direction in sequence, and a color image is formed by superposing (overlapping) the four monpchromatic images. In these methods, the method for arranging a plurality of image forming units in the paper feed direction (referred to as tandem method) is one of the best methods in that the apparatus is high in printing and recording speed and small in size.
FIG. 2 shows an example of the construction of the tandem color image forming apparatus of electrophotographic type in which four monochromatic (yellow, magenta, cyan and black) image forming units 6, 7, 8 and 9 are arranged in the paper feed direction (in the arrow direction). The respective image forming unit is composed of a photosensitive drum 6a, 7a, 8a or 9a (image recording medium), a print bar 6c, 7c, 8c or 9c (image recording means) for forming a latent image on the surface of the drum, a developing unit 6d, 7d, 8d or 9d for developing the latent image to a toner image, a transcribing roller 6e, 7e, 8e or 9e for transcribing the toner image on the drum onto a paper (transcribing medium), a charge removing lamp 6g, 7g, 8g or 9g for removing charge on the surface of the drum after transcription, a memory removing brush 6h, 7h, 8h or 9h for averaging the residual toner after transcription, and a charging unit 6b, 7b, 8b or 9b for charging the drum surface uniformly.
In the example shown in FIG. 2, although a cleaner-less process for returning the residual toner on the drum (after transcription) to the developing unit is shown, there exists another apparatus such that a cleaning unit is provided for cleaning the residual toner on the drum (after transcription), instead of the memory removing brush provided for the clearless process.
The toner images formed by the respective image forming units are transcribed being superposed with each other in sequence onto the paper fed by the transcription belt 10 under the condition that the paper is adhered to the transcription belt 10 by an electrostatic force generated therebetween. Further, the transcribed toner images are melted on and pressed against the paper by a fixing unit 12 to form a color image.
In the above-mentioned image forming apparatus, since a plurality of image forming units are provided as described above, there exists such an advantage that the time required to form a color image is relatively short, as compared with that required for the other methods. In contrast with this, however, since the four monochromatic images are transcribed onto a paper under overlapped conditions by the respective image forming units, the respective images tend to mismatch from each other (mal-register), with the result that there exists a problem in that the image quality is deteriorated. Here, "mal-register" implies that a mismatching of successive colors transcribed over each other.
The mechanism of occurrence of the mal-register in the color image obtained by the tandem type apparatus will be explained simply with reference to FIG. 3, which shows changes in mismatch of the respective monochromatic images from the respective ideal positions on paper. For brevity, only the mal-register in the paper feed (the sub-scanning) direction is taken into account, without consideration of the mal-register in the main scanning direction perpendicular to the paper feed direction. In the drawing, the mal-register waveform can be classified roughly into long-period fluctuations (whose amplitude is denoted by .alpha.), a short-period fluctuations (whose amplitude is denoted by .beta.), and an offset denoted by e in each of the four monochromatic images.
The long-period fluctuations are caused by the speed fluctuations due to eccentricities of the driving and transmitting mechanisms for the photosensitive drum, the transcribing belt, etc.; the short-period fluctuations are caused by the vibrations of the driving and transmitting mechanisms for the photosensitive drum, etc.; and the offset is caused by the mounting errors of the respective image forming units, change in distance between the units due to thermal expansion, the mounting error of the print bar, etc.
Therefore, the long-period fluctuations and the offset result in the mal-register when the monochromatic images are transcribed being superposed with each other, so that the color image is unstable in color reproducibility and resolution.
The allowable mal-register due to superposed images is about 1.5 times of the resolution in general. For instance, in the case of the printing apparatus of 300 dpi in resolution, the allowable mal-register is 0.127 mm. When this allowable value is sheared to the manufacturing precision of the respective elements, the respective elements must be manufactured on the order of several micrometers. For instance, in order to suppress the mal-register in color image due to the expansion and contraction of the formed images caused by the eccentricity of the rotating elements (e.g., gears), the concentricity of the gear (which exerts the most serious influence upon the mal-register) must be determined on the order of several micrometers. Further, it is necessary to mount the image forming units and the print bars at the precision of less than 20 micrometers.
On the other hand, although the short-period fluctuations are caused by the vibrations of the driving and transmitting mechanism for the photosensitive drum, even if the vibration amplitude is small, stripe-shaped color shade non-uniformity (referred to banding) develops in color image. In particular, this banding becomes prominent in the image of medium gradation. Therefore, in the case of the color printing apparatus, since photographs or images including many intermediate gradations are often printed, the image having the bandings is very unsightly, so that the banding must be suppressed as much as possible. In order to eliminate the banding, the rotative speed fluctuations of the photosensitive drum must be reduced below 1 to 3%, so that an extremely high precision is required to manufacture the driving mechanism of the photosensitive drum.
To eliminate the mal-register and the banding to such a low level as to be negligible, a high precision has been so far required for the manufacturing and assembling process. Further, various methods have been adopted as follows: high precisely formed worm gears are used to drive the four photosensitive drums (Electro-photography Proceedings, JAPAN HARDCOPY 1991 A-27 p); a mark is described on the surface of the transcribing belt to determine the image write timing (Japanese Published Unexamined (Kokai) Patent Application No. 62-242996); the distance between the image forming units is determined to be an integer times longer than the circumferential length of the pulley for driving the transcription belt (U.S. Pat. No. 4,531,828), etc. Further, the method of deciding the image writing timing by detecting the paper end is disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 59-163971.
However, there exists a limit with respect to an improvement in the mechanical precision when the manufacturing cost is taken into account. Further, even if the precision is increased up to the practicable possible degree, there still exists a problem in that the mal-register due to superposed images exceeds the allowable value, or when the units are exchanged with another one by the user due to change in ambient temperature for instance, since the distance between the image forming units changes, the mal-register rate in the color image inevitably increases.
Further, in the above-description, although the tandem color image forming apparatus has been explained by way of example, in the case where the single image forming unit of the color image forming apparatus functions as a monochromatic image forming apparatus (not as the color image forming apparatus), there exists the same common problem as in the color image forming apparatus. That is, when the rotative speed of the photosensitive drum fluctuates or the photosensitive drum has an eccentricity, the banding occurs even in the monochromatic image forming apparatus.
As described above, in the conventional monochromatic or color image forming apparatuses, in order to suppress the shade non-uniformity in the monochromatic image and the mal-register in the color image formed by the image forming apparatus, the manufacturing precision and assembling precision of the composing elements have been both retained extremely high, so that there exists a problem in that a complicated fine adjustment work is required in the assembling process or it is impossible to reduce the shade non-uniformity in the monochromatic image or the mal-register in the color image below an unnoticeable level due to a limit of precision in the assembling work. In addition, since the mal-register has been reduced in dependence upon only the higher precision of the mechanical parts or elements, even if the initial mal-register can be reduced in the color image, there exists a problem in that the image quality is deteriorated with the passage of time due to occurrence of the wear or play in the sliding portions of the mechanical parts.
Further, in the case of the tandem color image forming apparatus, in order to suppress the mal-register caused when the monochromatic images formed by the respective image forming units are superposed, an extremely high precision has been so far required to manufacture and assemble the composing elements of the units, so that there exists a problem in that a complicated fine adjustment work is required in the assembling process or it is impossible to reduce the shade non-uniformity in the monochromatic image or the mal-register in the color images below an unnoticeable level due to a limit of precision in the assembling work. In addition, since the mal-register has been reduced in dependence upon only the higher precision of the mechanical parts or elements, even if the initial mal-register can be reduced in the color images, there exists a problem in that the image quality is deteriorated with the passage of time due to occurrence of the wear or play in the sliding portions of the mechanical parts.
Accordingly, it has been difficult to keep the conventional tandem image forming apparatus under excellent color image registration conditions for many hours, and in addition the manufacturing cost thereof has been high because of the highly precise mechanical parts. For the above-mentioned reasons, the color image forming apparatus has been so far used only as a printing unit for a full color copying machine which is high in cost and can be maintained by the aid of a service man.
In other words, since the current requirements of the color image forming apparatus are maintenance-free and lower-cost as compared with the copying machine, the conventional tandem method cannot be applied to the color printer apparatus.